feat: allow multiple values on relation columns

[he-is-talha]

Summary

• Add a Select multiple items option on relation columns (customSettings.allowMultiple)
• Store relation values as one cell row per related id (same pattern as usergroup)
• Update create/edit UI, cell display, validation, import parsing, and analytics formatting for single and multi relations

Closes #2952

🖼️ Screenshots

Notes

• API cell values for relation columns are now always int[] (legacy scalars are still accepted and normalized)
• No DB migration; existing single-value cells continue to work

🤖 AI (if applicable)

The content of this PR was partly using AI

[samin-z]

@he-is-talha please clean you second commit message before merging.

[he-is-talha]

@samin-z Cleaned the second commit message and force-pushed.

[samin-z]

few points:

1. for multiple item when column is edited and it is switched to single item, table still shows multiple items until page is refreshed and then the correct value is shown.
2. editing multiple items and switching to single item, it shows no warning that existing data might be loss so maybe a nice addition to let users know, or show a warning underneath this option in regard to how toggling this will effect.
3. when the column is mandatory, clearing the relation still save with no error, it's better to make sure empty values are rejected both in the UI and on the API. same point for multi-selection, empty array shouldnt count as a valid value. Inline cell edit should also refuse to clear a mandatory relation. though i think this is pre-existing issue as if we have columns which rows which are empty and then the column is updated to mandatory but existing empty rows columns are still as they are.

for point 4 lets fix it for now for this case though.

[he-is-talha]

1. Implemented: Multi → single updates without refresh
2. Warning when turning off multi:

3. Mandatory rejects empty 4. Enabling mandatory with empty rows blocked

[he-is-talha]

@samin-z Ready for the review again

[Aveyron-RetD]

Maybe @Koc would be a good reviewer